I assume you somehow missed tutorial or just didn't get it. you need to come close to totems ( black cubic things on the map with white light boxes on them ) and press E to activate a session, then you listed to music played and after it is completed you try to replay it using keys 1 - 8 .
the game might be hard for the people who didn't really had any experience at music because you need to have good pitch to get what notes are played but failing is not a problem you always can press E once again, Listen to the music and try to "Repeat it"
I hope now you now how to play :) and thanks for feedback.